This invention relates to a resin composition, more particularly to a resin composition having a low volumetric shrinkage percentage at curing.
Generally, reins such as an unsaturated polyester resin, an acrylic resin, a phenol resin, an epoxy resin and the like undergo volumetric shrinkage at curing. This shrinkage causes defects of the appearance of the obtained material, and generates internal stress that causes cracks and other characteristic changes in (and even the destruction of) material sealed in the resin. Even in an epoxy resin which undergoes the least shrinkage among them, 3 to 5% of the volumetric shrinkage occurs and, by the shrinkage at curing, various kinds of problems have been caused.
Heretofore, in order to overcome these problems, for example, a method has been employed in which an inorganic filler such as silica, calcium carbonate or alumina has been added to the resin composition, thereby lessening the volumetric shrinkage of the resin. However, in this method, the viscosity of resin composition is remarkably increased by the addition of the filler so that it has the inconvenience that an injection operation and other manipulations at a low temperature become difficult. That the method can not be used to obtain moldings having good transparency also presents a problem.
Further, a method has been disclosed in which shrinkage stress at curing has been lessened by optimizing the conditions of the curing process, in that strain on the moldings is decreased lessen by gradually raising the curing temperature or the moldings are annealed over a long period of time after curing and then the curings are taken out. In this case, however, there is also the defect that more complicated process has been required. Accordingly, conventional methods present no fundamental solution, and it is impossible to remove or remarkably reduce the shrinkage at curing of the aforesaid resins.
On the other hand, as the other method as mentioned above, investigations were made to lower the shrinkage by a resin alone. For example, Kobunshi, 27, p. 108, (1978) discloses an 1,4,6-trioxaspiro[4,4]nonane. When this compound is made to be bifunctional, it can be used as a thermosetting resin, as mentioned in J. Macromol. Sci. Chem. A9, p. 849 (1975) and J. Polym. Sci. Polym. Symp, 56, p. 117 (1976).
According to these journal articles, to a bisspiroester compound, having a skeltal structure which comprises hydroquinone or bisphenol A, trifluoroborate-diethyl ether complex was added as a catalyst, and there were carried out a ring-opening curing reaction at room temperature for 24 hours. According to this method, by using a strong catalyst such as trifluoroborate-diethyl ether complex, the life of the resin composition is short and handling thereof is difficult, and corrosiveness to a metal is remarkable, but it is possible to make shrinkage of the resin zero at curing.